A great deal of research has been and is being conducted on the use of ceramics in the fabrication of structural components which must posess good mechanical properties at high temperatures. Of particular interest is silicon nitride because it exhibits high strength at elevated temperatures and excellent thermal shock, creep, and oxidation resistance in hostile atmospheres.
In fabricating structural components from silicon nitride, the usual procedure is to hot press or pressure sinter the silicon nitride, which is in finely divided form, in admixture with a sintering aid, such as magnesium oxide or alumina. In order to obtain a structural component having optimum physical properties, it is necessary that the silicon nitride be of very high purity and in finely divided form, i.e., of submicron size. Furthermore, alpha phase silicon nitride powder should be used as the starting material because higher strength and more uniform structures are obtained by hot pressing than when beta phase silicon nitride is present in any appreciable amount in the starting material.
In the past, a variety of procedures have been followed in synthesizing silicon nitride. These include the nitridation of silicon with ammonia and the reaction of silicon tetrachloride or tetrabromide with ammonia. In the latter method, silicon tetrachloride and ammonia are separately introduced into a furnace wherein the materials react at an elevated temperature. The silicon diimide that is formed decomposes, yielding silicon nitride which deposits on a substrate as a thin film. The former method involves charging ammonia to a furnace containing a mass of silicon and reacting the materials at an elevated temperature to form silicon nitride. The silicon nitride product is in the form of a spongelike material which prior to use is ball milled, employing iron or tungsten balls so as to provide silicon nitride powder. This method for preparing the powder is unsatisfactory in that the product is a mixed alpha and beta phase silicon nitride. Furthermore, the product is contaminated with excess silicon, boron, calcium, and transition elements as major impurities.
It is an object of this invention, therefore, to provide a method for preparing high purity, submicron, alpha phase silicon nitride powder.
Another object of the invention is to provide a silicon nitride powder which is particularly suitable for use in fabricating structural components.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure and the drawing which shows the infrared spectrum of silicon nitride powder prepared by the method of this invention.